Development of a microwave-enhanced isotopic labelling procedure based on the Eschweiler-Clarke methylation reaction

Article abstract of DOI:10.1016/S0040-4039(02)02455-3

A number of primary and secondary amines have been rapidly methylated under microwave-enhanced conditions using formic acid-formaldehyde mixtures, providing a route to ²H(D)-containing compounds and the potential for ³H(T), ¹¹C, ¹³C and ¹⁴C labelling.

Full text of DOI:10.1016/S0040-4039(02)02455-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9487–9488
Development of a microwave-enhanced isotopic labelling
procedure based on the Eschweiler–Clarke methylation reaction
John R. Harding,a John R. Jones,b,* Shui-Yu Lub and Robin Wooda
aAstraZeneca UK Ltd, Mereside, Alderley Park, Macclesfield, Cheshire SK10 4TG, UK
bDepartment of Chemistry, University of Surrey, Guildford, Surrey GU2 7XH, UK
Received 25 September 2002; accepted 1 November 2002
Abstract—A number of primary and secondary amines have been rapidly methylated under microwave-enhanced conditions using
2
3
formic acid-formaldehyde mixtures, providing a route to H(D)-containing compounds and the potential for H(T), 11C, 13C and
14C labelling. © 2002 Elsevier Science Ltd. All rights reserved.
All of the most widely used deuteriation/tritiation pro-
cedures—hydrogen isotope exchange,1,2 hydrogena-
tion,3 aromatic dehalogenation,4,5 and borohydride
reduction6—with the exception of methylation, have
benefited from the introduction of microwave-enhanced
procedures.7,8 Faster, cleaner reactions can be achieved
without the need for a gas handling facility when the
familiar donors D2 and T2 are replaced by solid, polar
donors such as formate. Furthermore, in the case of
tritium the amount of radioactive waste produced can
be greatly reduced and, if need be, converted to useful
by-products on further microwave treatment.9
the preparation of the corresponding deuteriated com-
pounds for amines 4–5. This represents the first applica-
tion of the microwave-enhanced reaction in the
labelling area (Scheme 1).
Three different types of instruments were used—the
Smith Creator focused MW synthesiser, the Synthe-
wave S402 monomode reactor and a Matsui BT 169
oven. Whilst reactions were carried out sequentially
using the first two devices, parallel reactions can be
studied using the latter. In a typical experiment, trans-
cinnamyl piperazine (16 mg, 0.08 mmol), HCHO (6 ml,
37% aqueous solution, 0.08 mmol), HCOOH (3 ml, 0.08
mmol) together with DMSO (0.2 ml) were mixed in a 5
ml Pyrex glass tube. The tube was subjected to
microwave irradiation at 120 W for 1 min. The reaction
mixture was dissolved in CHCl3 (2 ml). The chloroform
solution was washed with NaOH (10% aqueous solu-
tion, 6 ml) and dried over Na2SO4. Removal of solvent
by blowing a stream of N2 gas afforded methylated
Methyl iodide, despite its many disadvantages (a low
boiling liquid of poor stability and accompanying
health hazard, especially if it is radioactive), is still the
most widely used methylating agent. Our thoughts
therefore turned towards a replacement made up of
both a primary and secondary donor.
The combination of formic acid and formaldehyde
methylation of amines—known as the Eschweiler–
Clarke reaction,10,11—is a method of wide synthetic
utility and has been the subject of critical
investigation12 in which a series of substituted benzyl-
amines were used as model compounds. This, together
with a recently reported study13 in which the reaction
was subjected to microwave irradiation prompts us to
report our findings using amines 1–3 as model com-
pounds for non-isotopically labelled reactions and on
Keywords: deuterium; labelling; methylation; microwaves.
* Corresponding author. Tel.: +44 1483 689313; fax: +44 1483
686851; e-mail: j.r.jones@surrey.ac.uk
Scheme 1.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02455-3

9488
J. R. Harding et al. / Tetrahedron Letters 43 (2002) 9487–9488
products as a colourless oil (16 mg, 93%). The products
were characterised by H NMR spectroscopy and mass
spectrometry.
(d) The final compound to be methylated is desmethyl
Tamoxifen (5) to produce Tamoxifen-d3, a drug that
has found wide use for the treatment of cancer and
where the labelled form has many applications in
metabolism and pharmacokinetic studies.
1
The salient features of the results are as follows:
(a) In all cases methylation was extremely rapid and
was usually complete within 1–3 min. This contrasts
with the 24 h at 80°C that was used for the thermal
methylation.12 Whilst the ratio13 of formaldehyde to
amine and that of formic acid to amine had previously
been in the range 1.5–4, the present 1:1:1 ratio does
away with the need for excess reagents; this will be
particularly advantageous for radiochemical work.
(e) Simple modification of the methylation procedure
3
would allow H compounds to be prepared. Similarly
compounds containing the –13CD3 group, for which
there is an increasing demand, could be easily synthe-
sised. Finally, the rapidity of the methylation reaction
and the increasing availability of key 11C building
blocks suggest new opportunities in the rapidly expand-
ing PET radiopharmaceutical area.15
(b) The reactions could be performed in the absence of
solvent when the amine was soluble in the formic
acid–formaldehyde mixture, failing this DMSO was
used.
In summary, we have developed an extremely rapid
microwave-enhanced methylation procedure using a
combination of primary and secondary donors that has
initially been used for deuterium labelling, but which
has considerable potential for incorporating other iso-
topes, both stable and radioactive.
(c) As for the earlier hydrogenation work3 the deu-
terium distribution in the primary and secondary
donors could be easily varied thereby providing a con-
venient route to three isotopomers (Scheme 2). Formal-
dehyde supplies the N-Me group with two deuterium
atoms whilst the formic acid provides one. Only when
the fully deuteriated mixture—DCDO+DCO2D—is
employed are three deuterium atoms incorporated.
Early work14 with 14C had also established that the
carbon of the N-Me group derived from the aldehyde.
No isotope exchange between DCDO and HCOOH or
HCHO and DCOOD was observed.
References
1. Barthez, J. M.; Filikov, A. V.; Frederiksen, L. B.;
Huguet, M.-L.; Jones, J. R.; Lu, S. Y. Can. J. Chem.
1998, 76, 726–728.
2. Anto, S.; Getvoldsen, G. S.; Harding, J. R.; Jones, J. R.;
Lu, S. Y.; Russell, J. C. J. Chem. Soc., Perkin Trans. 2
2000, 2208–2211.
3. Al-Qahtani, M. H.; Cleator, N.; Danks, T. N.; Garman,
R. N.; Jones, J. R.; Stefaniak, S.; Morgan, A. D.; Sim-
monds, A. J. J. Chem. Res. (S) 1998, 400–401.
4. Jones, J. R.; Lockley, W. J. S.; Lu, S. Y.; Thompson, S.
P. Tetrahedron Lett. 2001, 42, 331–332.
5. Chappelle, M. R.; Kent, B. B.; Jones, J. R.; Lu, S. Y.;
Morgan, A. D. Tetrahedron Lett. 2002, 43, 5117–5118.
6. Erb, Th. W.; Jones, J. R.; Lu, S. Y. J. Chem. Res. (S)
1999, 728–729.
7. Elander, N.; Jones, J. R.; Lu, S. Y.; Stone-Elander, S.
Chem. Soc. Rev. 2000, 29, 239–249.
8. Jones, J. R.; Lu, S. Y. In Microwaves in Organic Synthe-
sis; Loupy, A., Ed.; Wiley-VCH: Chichester, 2002, in
press.
9. Jones, J. R.; Langham, P. B.; Lu, S. Y. Green Chem.
2002, 5, in press (DOI:10.1039/b204176f).
10. Eschweiler, W. Chem. Ber. 1905, 38, 880.
11. Clarke, H. T.; Gillespie, H. B.; Weisshaus, S. Z. J. Am.
Chem. Soc. 1933, 55, 4571–4587.
12. Pine, S. H.; Sanchez, B. L. J. Org. Chem. 1971, 36,
829–832.
13. Torchy, S.; Barby, D. J. Chem. Res. (S) 2001, 292–293.
14. Tarpey, W.; Hauptmann, H.; Tolbert, B. M.; Rapoport,
H. J. Am. Chem. Soc. 1950, 72, 5126–5127.
15. Hernandez, L.; Casanova, E.; Loupy, A.; Petit, A. J.
Labelled Comp. Radiopharmaceuticals 2002, in press.
Scheme 2.